Memory that retains its data even when operation power is not available is classified as non-volatile memory (NVM). Examples of non-volatile memory are nvSRAM, ferroelectric RAM (F-RAM), programmable-read-only memory (PROM), erasable-programmable-read-only memory (EPROM), electrically-erasable-programmable-read-only-memory (EEPROM), and flash memories. Some memory arrays may include multiple memory cells that utilize transistors and gate structures. Such memory cells may include a charge trapping layer, such as a nitride layer or an oxynitride layer. The charge trapping layer may be programmed to store data based on voltages applied to or received by the memory array. This class of memory may be used in applications in which critical data must be stored after power is removed, or when power is interrupted during operation.
The memory cells may be programmable to one of the two binary states, programmed or erased. In some embodiments, a programmed cell may be assigned a bit value “0” and an erased cell a bit value “1”. In other embodiments, the assignment may be reversed. In single-ended sensing configuration,each memory cell stores one bit of data. A binary state value of the cells, such as threshold voltage (Vt) or cell current (I), may be compared to a reference value to determine the binary state (programmed or erased) of the cell. In differential sensing configuration, two memory cells (true or target and compier twin cells) may form a complementary pair to store one bit of data. The bit value of the complementary pair corresponds to the state of the target cell. The binary state value of the target cell may be compared to the twin cell to determine the binary state of the complementary pair.
Memory cells may also be multi-level storage cells in which each memory cell may store more than one bits of data.